The present invention relates to external storage devices for storing data in a computer system and to their integrated system, and more particularly to remote copy techniques for mirroring data between remote external storage devices (disk subsystems) without involving an upper hierarchical apparatus or host computer, by interconnecting remote external storage devices and other remote external storage devices. The disk subsystem is herein intended to mean a control unit for controlling data transfer to and from an upper hierarchical apparatus and a storage device having disks for storing data or a storage device having an internal buffer.
External storage systems incorporating a so-called remote copy function have already been in practical use, in which data is mirrored and stored in disk subsystems of a main center and a remote center.
Such prior art has various issues to be solved because the remote copy function is realized by involving host computers.
“Synchronous Type and Asynchronous Type”
The remote copy function is mainly classified into two types, a synchronous type and an asynchronous type.
The synchronous type executes the following process sequence. When a disk subsystem is instructed by a host computer (upper hierarchical apparatus) of a main center to renew (write) data and if the disk subsystem is assigned the remote copy function, a renewal process completion notice is issued to the host computer of the main center only after the instructed data renewal (write) is completed for a corresponding disk subsystem in a remote center. A time delay (transmission time and the like) is generated in accordance with a geographical distance between the main center and remote center and the performance of a data transmission line therebetween. If the transmission time of the synchronous type is taken into consideration, several tens Km is a practical limit of a distance to a remote site.
In the synchronous type, the data contents in disk subsystems in the main and remote centers are always consistent from a macro viewpoint. Therefore, even if the function of the main center is lost by accidents or the like, the data contents immediately before the accidents are perfectly retained in the disk subsystems of the remote center and the process can be resumed quickly at the remote center. The term “always consistent from the macro viewpoint” means that during the execution of the synchronous type function, although the data contents may be different in terms of a process time (•sec, msec) of magnetic disk devices and electronic circuits, the data contents are always the same at the time of data renewal completion. This is because the renewal process at the main center cannot be completed unless the renewal data is completely reflected upon the remote center. Therefore, in some cases, particularly if a distance between the main and remote centers is long and the data transmission line is congested, the access performance to a disk subsystem in the main center is considerably degraded.
In contrast, the asynchronous type executes the following process sequence. When a disk subsystem is instructed by a host computer of a main center to renew (write) data and even if this data is to be remotely copied, a renewal process completion notice is issued to the host computer of the main center immediately after the data renewal process for the disk subsystem in the main center is completed, to thereafter execute the data renewal (reflection) of the disk subsystem in the remote center, asynchronously with the data renewal in the main center. Since the data renewal is completed in a process time required by the main center, there is no transmission delay time or the like to be caused by storing the data in the remote center.
In the asynchronous type, the data contents in a disk subsystem of the remote center are not always consistent with those in the main center. Therefore, if the function of the main center is lost by accidents or the like, the data still not reflected upon the remote center is lost. However, an access performance of a disk subsystem in the main center can be maintained at the level when the remote copy function is not executed.
In order to back up data so as not to be lost by natural disasters such as earthquakes, it is necessary to set the distance between the main and remote centers to about 100 km to several tens km. Although it is possible to use a high speed communication line, for example, of a 100 Mbit/sec to 300 Mbit/sec class for the remote copy function, an expensive line subscription fee is incurred upon a customer of the disk subsystem, and this approach is not economically suitable.
“Order Integrity”
There is another problem different from the above-described issue of the data transmission time. Namely, if the remote center backs up the data of a plurality of disk subsystems of the main center, there occurs an issue (order integrity) that disk subsystems are required to be in one-to-one correspondence. In asynchronous remote copy, it is inevitable that reflection of renewal data in the remote center is delayed from the time when an actual renewal process is executed in the main center. However, the order of renewal in the remote center is required to be the same as that in the main center.
A database or the like is generally constituted of a main body of the database and various log and control information directly associated with the main body. When data is renewed, not only the database main body but also the log and control information is renewed to maintain the system integrity. Therefore, if the renewal order is not kept, the integrity of information regarding the renewal order is also lost, and at the worst the whole of the database may be destructed.
“Involvement of Host Computer”
In the asynchronous remote copy under general environments where the main and remote centers have a plurality of disk subsystems, when the host computer instructs the disk subsystem to renew data, it is common that the host computer adds renewal order information such as a time stamp to the data to make the corresponding disk subsystem in the remote center execute a renewal data reflection process in accordance with the added information.
According to the remote copy function disclosed, for example, in the publication of JP-A-6-290125 (U.S. Pat. No. 5,446,871), generation and supply of renewal order information and a renewal data reflection process based upon this information are realized through the cooperation between the operating system of a host computer in a main center and its disk subsystems and the operating system of a host computer in a remote center and its disk subsystems.